1. Field of the Invention
The present invention relates to an electrode for a nonaqueous electrolyte secondary battery, and a nonaqueous electrolyte secondary battery in which the same is used.
2. Description of the Related Art
A lithium-ion secondary battery includes, between a positive electrode and a negative electrode, a separator which electrically insulates the respective electrode plates and further serves to retain an electrolyte solution. Currently, organic polymer-type microporous films composed of polyethylene, polypropylene, or the like are mainly used as separators.
A microporous film composed of the above resins has a property such that it shrinks at a temperature of about 100° C. Therefore, when a metallic element such as lithium has deposited due to peeling of a portion of an active material layer or the like, or when a protrusion having a sharp shape such as a nail has penetrated the battery from the outside, a short-circuit current flows, and the heat of reaction which instantaneously occurs due to the flowing current causes the separator to shrink. When the separator shrinks, the short-circuiting area between the positive electrode and the negative electrode is further expanded.
As a measure of preventing short-circuiting, Japanese Laid-Open Patent Publication No. 7-220759 discloses a method of forming a porous protection film on the surface of a negative-electrode active material layer or a positive-electrode active material layer. This porous protection film is formed by mixing a powder of inorganic substance such as alumina with a binder agent of resin, and applying the mixture onto the surface of the negative-electrode active material layer or the positive-electrode active material layer.
Japanese Patent No. 3371301 discloses a technique of forming a porous coating film through application on the surface of an active material layer, the porous coating film containing solid microparticles.
Japanese Laid-Open Patent Publication No. 6-36800 discloses a technique of forming a porous insulating film on the surface of a negative electrode by sputtering. Japanese Laid-Open Patent Publication No. 6-36800 describes that, when forming a porous insulating film by a sputtering technique, the thickness is desirably 5 nm to 100 nm.
When a porous protection film is formed by coating technique as is disclosed in Japanese Laid-Open Patent Publication No. 7-220759 and Japanese Patent No. 3371301, there is a problem in that the porous protection film has poor adherence.
Moreover, an active material layer is formed by applying a mixture of an active material and an organic solvent containing a binder (binder agent) onto a current collector. Therefore, if a porous protection film in which an organic solvent containing a binder is mixed is applied further thereon, one layer may dissolve into the organic solvent of the other layer such that the active material layer and the porous protection film become mixed together. In order to prevent this, as a solvent to be used for the paint to be applied as the overlayer, it has been necessary to employ a separate organic solvent to which the underlying binder is unlikely to dissolve. Even if such an organic solvent is used, the electrode plates may be warped due to a difference in their shrinkage ratios when being dry.
On the other hand, a porous insulating film which is formed by the sputtering technique of Japanese Laid-Open Patent Publication No. 6-36800 has a strong internal stress. Therefore, with the method of Japanese Laid-Open Patent Publication No. 6-36800, it is difficult to form a film so thick as to provide sufficient insulation between the positive electrode and the negative electrode. There is also a problem in that, since a film having a high density is formed by sputtering technique, the ion conductivity between the positive electrode and the negative electrode is lowered.
The present invention has been made in order to solve the above problems, and an objective thereof is to provide a nonaqueous electrolyte secondary battery which is excellent in safety because short-circuiting is unlikely to progress, this being achieved by enhancing the adherence between an active material layer and an inorganic insulating layer while ensuring electrical insulation and ion conductivity between electrodes, and provide an electrode for a nonaqueous electrolyte secondary battery.